1. Technical Field
The invention relates generally to multi-directional viewing and imaging, and more specifically, to a solution that enables viewing and/or imaging from any and all directions with few or no moving parts.
2. Background Art
In general, there are four approaches to obtaining a panoramic view of an area. In a first approach, a rotating camera is used. This approach is used in many areas to cover both full panoramas as well as fields of view of less than three hundred sixty degrees. When compared to other approaches, the rotating camera is relatively low cost, simple, provides intuitive operation, provides a known performance, is a known technology, provides minimal distortion of the imaged area, and can provide excellent resolution. Additionally, the rotating camera provides a single point-of-view, which can be important in certain applications, such as machine vision or targeting. However, the rotating camera is limited by the requirement to rotate the camera, an inability to view the entire panorama at once, a need to assemble (e.g., “stitch together”) individual images to obtain the full panorama, and a time limitation on how quickly a particular portion of the field of view may be accessed on demand.
In a second approach, a camera cluster (e.g., multiple cameras whose fields of view are combined) are used to produce a single panoramic image. For example, ten cameras, each having a field of view that covers approximately one-tenth of the panorama, theoretically can be used to view the entire panorama at once. In practice, some overlap is generally included in the fields of view. This approach provides the advantages of an intuitive design, provides a known performance, is a known technology, provides minimal distortion of the images area, and can provide excellent resolution. However, this approach also requires the assembly of multiple images, has a higher cost compared to the rotating camera, and each camera comprises a unique point of view.
In a third approach, a panoramic (e.g., “fish-eye”) lens is used. The panoramic lens increases the field of view of a particular camera, thereby reducing a number of cameras needed or a required amount of movement to obtain a panoramic view. Additionally, use of the panoramic lens enables a more simplistic solution, and avoids blind spots in the field of view. However, panoramic lenses generally are more expensive, are relatively heavy and bulky as compared to standard optics, and images having relatively large fields of view include greater distortion, particularly at the edges. The distortion effectively creates multiple points of view in most panoramic lens-based solutions. Additionally, using the same imaging hardware, panoramic lens-based solutions have inherently inferior resolution to that of a camera cluster or a rotating camera since the entire panorama is mapped to a single imaging device.
In a fourth approach, one or more mirrors are used to increase the field of view. In general, this approach requires no moving parts, a simultaneous apprehension of the entire panorama, and a physically simple design. For example, one mirror-based approach uses multiple planar mirrors in conjunction with multiple charge coupled device (“CCD”) cameras to obtain a full panoramic image. In this approach, four planar mirrors are arranged in a pyramidal shape with one camera positioned above each of the four planar mirrors to obtain the panoramic image. However, this approach requires multiple cameras and suffers from distortion at the “seams” when the separate images are combined to yield the panoramic view.
Other mirror-based approaches have used curved mirrors in conjunction with image sensors to provide an omnidirectional view. Hyperbola and ellipsoid reflective surfaces possess a single viewpoint in perspective projection when carefully designed and implemented. One approach provides a conical projection image sensor (COPIS) that uses a conical reflecting surface to gather images from the surrounding environment. The images can be processed to guide the navigation of a mobile robot. Although COPIS is able to attain full panoramic viewing, it is not a true omnidirectional image sensor since the field of view is limited by the vertex angle of the conical mirror and by the viewing angle of the camera lens. Furthermore, reflection off the curved surface results in multiple viewpoints, as the locus of viewpoints for a cone is a circle. Multiple viewpoints cause significant distortion and require complex manipulation and translation of the image to reconstruct the scene as viewed from a single viewpoint.
A proposed improvement to COPIS uses a hyperboloidal mirror in place of the conical surface. In this case, the rays of light which are reflected off the hyperboloidal surface, no matter where the point of origin, converge at a single point, thus enabling perspective viewing. Although the use of a hyperboloidal mirror enables full perspective image sensing, since the rays of light that make up the reflected image converge at the focal point of the reflector, the position of the sensor relative to the reflecting surface is critical, and any disturbance will impair the image quality. Further, the use of a perspective projection model inherently requires that as the distance between the sensor and the mirror increases, the cross section of the mirror must increase. Therefore, practical considerations dictate that in order to keep the mirror at a reasonable size, the mirror must be placed close to the sensor. This in turn causes complications with respect to the design of the image sensor optics. In addition, mapping the sensed image to usable coordinates requires complex calibration due to the nature of the converging image.
As a result, a need exists for an improved omnidirectional viewing/imaging solution that addresses one or more of these limitations and/or other limitation(s) not expressly discussed herein.